Method for desulphurizing a hydrocarbon mixture

ABSTRACT

Process for the desulphurization of a mixture of hydrocarbons containing sulphur compounds, comprising a step of oxidation by means of hydrogen peroxide, acetic acid and an acid catalyst in order to oxidize the sulphur compounds, in which the acid catalyst contains an acid solid chosen from cation-exchange resins having a pKa of less than or equal to 4, from solids based on silicon oxide additionally containing a trivalent metal, and from particles of inorganic solid on which acidic organic groups are grafted such that the particles thus grafted have a pKa of less than or equal to 4.

[0001] The present invention relates to a process for the desulphurization of a mixture of hydrocarbons containing sulphur compounds, comprising at least one oxidation step, in which there are used, as oxidizing agent, hydrogen peroxide, acetic acid and an acid catalyst, so as to oxidize the sulphur compounds.

[0002] For environmental reasons, specifications of the sulphur content of fuels are becoming increasingly strict. Thus, since the year 2000, the maximum sulphur contents in petrol and diesel allowed by the European Union are 150 and 350 ppm by weight, respectively. From 2005, these limits will drop to 50 ppm by weight. Restrictions are also expected for heating fuel oil, for which the current specification is 2000 ppm by weight of sulphur.

[0003] The conventional process for removing sulphur from petroleum products is based on the hydrodesulphurization reaction represented by

RSR′+2H₂→RH+R′H+H₂S

[0004] where RSR′ represents an aliphatic or aromatic sulphur compound.

[0005] This known process has certain disadvantages. For example, compliance with new specifications on sulphur requires stricter hydrodesulphurization conditions (excess of hydrogen, higher temperature, higher pressure, and the like) and necessarily causes an increase in the cost of fuels. In addition, some sulphur compounds which are found in petroleum cuts, such as benzothiophenes and substituted dibenzothiophenes such as for example 4-methyldibenzothiophene and 4,6-dimethyl-dibenzothiophene, are very resistant to hydrodesulphurization. The sulphur present therein is therefore difficult to remove by this route.

[0006] An alternative to the hydrodesulphurization process based on the oxidation of sulphur compounds is described in the document entitled “DMSO extraction of sulfones from selectively oxidized fuels” which was published in “Symposium on General Papers Presented Before the Division of Petroleum Chemistry, Inc., 217^(th) National Meeting, American Chemical Society, Anaheim, Calif., Mar. 21-25, 1999”. It is an oxydesulphurization by means of hydrogen peroxide, acetic acid and sulphuric acid in order to convert the sulphur compounds to corresponding sulphones, followed by extraction treatments in order to remove the sulphones. The sulphuric acid catalyses the formation of peracetic acid by reaction between hydrogen peroxide and acetic acid.

[0007] This known alternative has certain disadvantages because it could lead to losses in sulphuric acid. On the one hand, these losses could result in solubilization of H₂SO₄ in the organic phase essentially consisting of fuel, which would lead to a possible contamination of the fuel with residues of sulphuric acid, a substance containing sulphur, an element which it is precisely desired to remove. On the other hand, these losses could result in acid-base reactions with certain nitrogen compounds present in the fuel, such as quinolines and acridines, which would lead to the formation of sulphates not capable of catalysing the formation of peracetic acid.

[0008] The present invention aims to avoid the abovementioned disadvantages and to provide a novel process for removing sulphur from mixtures of hydrocarbons which makes it possible to significantly reduce their sulphur content.

[0009] The invention therefore relates to a process for the desulphurization of a mixture of hydrocarbons containing sulphur compounds, comprising a step of oxidation by means of hydrogen peroxide, acetic acid and an acid catalyst in order to oxidize the sulphur compounds, in which the acid catalyst contains an acid solid chosen from cation-exchange resins having a pKa of less than or equal to 4, from solids based on silicon oxide additionally containing a trivalent metal, and from particles of inorganic solid on which acidic organic groups are grafted such that the particles thus grafted have a pKa of less than or equal to 4.

[0010] The expression “pKa” is understood to mean -log Ka where Ka is the dissociation constant of the acid in aqueous medium at 25° C.

[0011] In the process of the invention, the acid catalyst is necessary to accelerate the formation of peracetic acid starting with hydrogen peroxide and acetic acid by the reaction [1]

CH₃CO₂H+H₂O₂⇄CH₃CO₃H+H₂O  [1]

[0012] The peracetic acid thus formed is the substance which oxidizes the sulphur compounds, in particular to corresponding sulphones, according to the reaction [2]

R′SR+2CH₃CO₃H→R′SO₂R+2CH₃CO₂H  [2]

[0013] Since following the oxidation, the acetic acid is reformed, the only reagent which is consumed in this process is the hydrogen peroxide.

[0014] In the reaction medium for oxidation, it is possible to distinguish at least two phases: an organic phase essentially consisting of the mixture of hydrocarbons and which may optionally contain part of the acetic acid and part of the peracetic acid, and an aqueous phase essentially containing water, hydrogen peroxide, part of the acetic acid and part of the peracetic acid.

[0015] One of the characteristic aspects of the invention lies in the use of a solid as catalyst to accelerate the formation of the peracetic acid.

[0016] In a first configuration, the solid catalyst may be in contact with the aqueous and organic phases in a three-phase medium, the third phase being solid and consisting of the solid catalyst. The catalyst may for example be used in suspension in the reaction medium or in the form of a fixed bed. In both cases, the fact that a solid is used has the advantage that the catalyst can be easily separated from the reaction medium and in particular from the organic phase, for example by filtration when the catalyst is in suspension. Because of this, contamination of the organic phase with the catalyst is avoided.

[0017] In a second configuration, the catalyst may be in contact with the aqueous phase alone, in a separate tank, for example in the form of a suspension, a fixed bed or a fluid bed. This second configuration has an additional advantage of avoiding side reactions between the catalyst and the organic phase, and in particular the acid-base reactions mentioned above.

[0018] The expression “mixture of hydrocarbons” is understood to mean any product predominantly containing combustible hydrocarbons such as paraffins, olefins, naphthenic compounds and aromatic compounds. This may be crude oil or a petroleum derivative obtained by any known refining, treatment. The mixture of hydrocarbons may be chosen from vehicle fuels such as petrol or diesel, and from domestic fuels such as, for example, heating fuel oil.

[0019] The expression “sulphur compounds” denotes all the compounds present in the mixture of hydrocarbons which contain sulphur. They are in particular benzothiophene, dibenzothiophene and their mono- or polysubstituted derivatives, more specifically 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene.

[0020] The expression “desulphurization” denotes any treatment which makes it possible to reduce the sulphur content of the mixture of hydrocarbons.

[0021] The sulphur compounds may be oxidized for example to corresponding sulphoxides, sulphones and sulphonic acids. In the case of 4,6-dimethyldibenzothiophene, the corresponding sulphoxide has the following structure

[0022] the corresponding sulphone has the following structure

[0023] and the corresponding sulphonic acid has the following structure

[0024] According to a first variant of the process of the invention, the acid catalyst contains a cation exchange resin having a pKa of less than or equal to 4. The pKa is advantageously less than or equal to 3, in particular 2, preferably 0 and most particularly less than or equal to −1.74.

[0025] In this first variant of the process according to the invention, the resin may be chosen from fluorinated resins, where the hydrogen atoms of the hydrocarbon backbone have been partially or completely substituted with fluorine atoms. Completely fluorinated resins are preferred. The resins preferably further contain acid groups. The acid groups may be chosen from phenol, arsonic, phosphonic, phosphinic, seleninic, selenonic, sulphinic, sulphonic and carboxylic groups. Sulphonic and/or carboxylic groups are preferred. Fluorinated resins containing sulphonic groups give good results. Typical structures of fluorinated resins carrying sulphonic groups are described in the publication by M. A. Harmer et al., J. Am. Chem. Soc. 1996, 118, 7708-7715 and in Kirk-Othmer, Encyclopedia of Chemical Technology, Fourth Edition, 1991, Volume 1, p. 970. There may be mentioned by way of example of a fluorinated resin having sulphonic groups, resins with the trademark NAFION® from the company DUPONT. It is also possible to use fluorinated resins having carboxylic groups. Typical structures of fluorinated resins carrying carboxylic groups are described in the publication by H. Ukihashi, CHEMTECH FEBRUARY 1980, 118-120 and in Kirk-Othmer, Encyclopedia of Chemical Technology, Fourth Edition, 1991, Volume 1, p. 970. There may be mentioned by way of example the resins with the trademark FLEMION® from the company ASAHI.

[0026] In this first variant, the acid catalyst may consist of the resin alone or it may consist of a matrix in which the resin is dispersed in the form of particles. The matrix is preferably inorganic. It most often contains silica, alumina, zirconium or titanium oxide. Pure silica is preferred. There may be mentioned by way of example the material NAFION® SAC 13 marketed by ALDRICH, the synthesis of which is described in the publication by M. A. Harmer et al., J. Am. Chem. Soc. 1996, 118, 7708-7715.

[0027] The quantity of acid catalyst used in the first variant of the process of the invention should be sufficient to allow rapid formation of peracetic acid. It depends on the content of acid sites of the solid acid catalyst and the nitrogen peroxide and acetic acid concentrations of the aqueous phase. The quantity of acid catalyst is such that the number of mol of protons from the acid groups such as the carboxylic and/or sulphonic groups, per kg of aqueous phase with which the catalyst is in contact, is generally greater than or equal to 0.001, in particular 0.005 and most particularly 0.01. The quantity of acid catalyst is most often such that the number of mol of protons per kg of aqueous phase is less than or equal to 0.8, in particular 0.5 and most particularly 0.2. Quantities of acid catalyst which lead to a number of mol of protons per kg of aqueous phase greater than or equal to 0.08 and less than or equal to 0.1 give good results.

[0028] According to a second variant of the process of the invention, the acid catalyst contains a solid based on silicon oxide which additionally contains a trivalent metal. The silicon/trivalent metal molar ratio in the solid is generally greater than or equal to 3, in particular 10. This ratio is usually less than or equal to 200, in particular 175. Values greater than or equal to 12 and less than or equal to 150 give good results. The trivalent metal may be chosen from boron, aluminium, gallium and iron. Aluminium is preferred. The solid is most often chosen from zeolites, clays and amorphous solids based on silicon oxide additionally containing a trivalent metal, in particular aluminium. The protonated forms of these solids are preferred. Zeolites of the H-ZSM-5, H-MOR, H-Beta and H-Y type gave good results. The zeolites H-ZSM-5 and H-MOR are preferred. On the other hand, titanium-containing zeolites give poor results. Consequently, the solid based on silicon oxide used in this variant of the process according to the invention is preferably free of titanium.

[0029] In this second variant of the process of the invention, the acid catalyst may consist of the solid alone or it may consist of the solid further containing organic functionalities. These may be chosen from sulphonic and carboxylic organic acid groups and from mixtures thereof.

[0030] The quantity of acid catalyst used in the second variant of the process of the invention should be sufficient to allow rapid formation of peracetic acid. It depends on the content of acid sites of the solid acid catalyst and the hydrogen peroxide and acetic acid concentrations of the aqueous phase. The quantity of acid catalyst is generally such that the number of mol of trivalent metal per kg of aqueous phase with which the catalyst is in contact is greater than or equal to 0.001, in particular 0.005 and most particularly 0.01. The quantity of acid catalyst is usually such that the number of mol of trivalent metal per kg of aqueous phase is less than or equal to 0.8 in particular 0.5 and most particularly 0.2. Quantities of acid catalyst which lead to a number of mol of trivalent metal per kg of aqueous phase greater than or equal to 0.08 and less than or equal to 0.1 give good results.

[0031] According to a third variant of the process of the invention, the acid catalyst contains inorganic solid particles on which acid organic groups have been grafted such that the particles thus grafted have a pKa of less than or equal 4. The pKa is advantageously less than or equal to 3, in particular 2, preferably 0 and most particularly less than or equal to −1.74.

[0032] In this third variant, the inorganic solid may be chosen from silicon, aluminium, zirconium and titanium oxide. Silicon oxide is preferred.

[0033] In the third variant, the acid organic groups may be chosen from aliphatic, alicyclic, heterocyclic or aromatic groups containing an acid functionality. The acid functionalities may be chosen from phenol, arsonic, phosphonic, phosphinic, seleninic, selenonic, sulphinic, sulphonic and carboxylic functionalities. Sulphonic and/or carboxylic functionalities are preferred. These groups may contain up to 18 carbon atoms, in particular up to 12 carbon atoms, preferably up to 6. They may additionally contain one or more heteroatoms such as oxygen and/or fluorine.

[0034] In the third variant, the acid organic groups may be grafted on the inorganic solid particles by any suitable known means, such as for example by the process described in the publication by J. H. Clark et al., C. R. Acad. Sci. Paris, Series IIc, Chimie/Chemistry 3 (2000) 399-404.

[0035] The quantity of acid catalyst used in the third variant of the process of the invention should be sufficient to allow rapid formation of peracetic acid. It depends on the content of acid sites in the catalyst and the hydrogen peroxide and acetic acid concentrations of the aqueous phase. The quantity of acid catalyst is generally such that the number of mol of protons from the acid organic groups per kg of aqueous phase with which the catalyst is in contact is greater than or equal to 0.001, in particular 0.005 and most particularly 0.01. The quantity of acid catalyst is usually such that the number of mol of protons per kg of aqueous phase is less than or equal to 0.8, in particular 0.5 and most particularly 0.2. Quantities of acid catalyst which give a number of mol of protons per kg of aqueous phase greater than or equal to 0.08 and less than or equal to 0.1 give good results.

[0036] In the oxidation step of the process according to the invention, the catalyst is generally used in the form of particles which may be obtained by any known process. The most diverse forms of particles come to mind, such as in particular powders, beads, pellets, extrudates or honeycomb structures. The average size of these particles depends on the type of use. For a process where the catalyst is in suspension, the average size of the particles is generally greater than or equal to 5 μm, more particularly 10 μm and most particularly 50 μm. The average size of the particles is usually less than or equal to 500 μm, more particularly 250 μm and most particularly 150 μm. Average sizes greater than or equal to 100 μm and less than or equal to 125 μm are particularly suitable. For a process where the catalyst is used in a fixed bed, the average size of the particles is generally greater than or equal to 0.5 mm, more particularly 1 mm and most particularly 2 mm. The average size of the particles is commonly less than or equal to 100 mm, more particularly 75 mm and most particularly 50 mm. Average sizes greater than or equal to 5 mm and less than or equal to 30 mm are particularly suitable.

[0037] In the process according to the invention, the oxidation is generally carried out at a temperature greater than or equal to 0° C., particularly 10° C. and preferably 20° C. The temperature is usually less than or equal to 100° C., in particular 90° C. and most particularly 80° C. Temperatures of 25 to 70° C. are suitable.

[0038] In the process of the invention, the hydrogen peroxide is generally used in the oxidation reaction in the form of an aqueous solution. Before mixing with the acetic acid, this solution most often has a hydrogen peroxide concentration greater than or equal to 1% by weight, in particular 10% by weight. The hydrogen peroxide concentration is commonly less than or equal to 80%, in particular 70%. A concentration of 30 to 60% by weight is suitable.

[0039] The quantity of hydrogen peroxide present in the oxidation reaction medium depends on the quantity of sulphur present in the mixture of hydrocarbons. The molar ratio between the hydrogen peroxide and the sulphur is generally greater than or equal to 1, in particular 2. This ratio is often less than or equal to 5 000, in particular 3 000. Ratios of 3 to 1 500 are particularly suitable.

[0040] In the process of the invention, the acetic acid is generally used in the oxidation reaction in the form of an aqueous solution.

[0041] The molar ratio between the acetic acid and the hydrogen peroxide used in the oxidation reaction medium is generally greater than or equal to 0.01, in particular 0.1 and most particularly 0.25. This ratio is often less than or equal to 4, in particular 2. Ratios of 0.5 to 1.5 are particularly suitable.

[0042] The aqueous phase entering into contact with the mixture of hydrocarbons is used in a volume such that the ratio of the volumes of this aqueous phase and of the mixture of hydrocarbons ensures optimum dispersion of the phases. This ratio is generally less than or equal to 0.5, in particular 0.3. It is usually greater than or equal to 0.01, in particular 0.05. Values greater than or equal to 0.1 and less than or equal to 0.25 are preferred.

[0043] In the process according to the invention, the oxidation may be carried out at atmospheric pressure or at supra-atmospheric pressure. It is preferable to work at atmospheric pressure.

[0044] The oxidation may be preceded by one or more hydrodesulphurization steps. It may also be followed by one or more steps for separating the oxidized sulphur compounds. These separations may be carried out in various ways: distillation, extraction by means of solvents, adsorption onto solids, pyrrolysis, acid or base hydrolysis and precipitation.

[0045] The process according to the invention may be carried out continuously or batchwise.

[0046] In a particularly advantageous embodiment of the process according to the invention, the process is carried out continuously using the plant schematically represented in the figure. In this plant, the oxidation reaction is carried out in the tank 1 which is fed with a mixture of hydrocarbons at the bottom of the tank by the pipe 2 and with hydrogen peroxide and peracetic acid solution at the top of the tank by the pipe 3. In the tank 1, two phases are distinguishable: an organic phase essentially consisting of the mixture of hydrocarbons, and an aqueous phase containing in particular hydrogen peroxide and peracetic acid. After oxidation, the oxidized organic phase which is less dense than the aqueous phase is removed at the top of the tank 1 by the pipe 4 and it may be transferred to a unit for separating the oxidized sulphur compounds. After oxidation, the aqueous phase containing in particular the hydrogen peroxide and the acetic acid leaves the tank 1 by the pipe 5 and is transferred into the tank 6, in which the solid acid catalyst is present. The latter remains inside the tank 6. In the latter, the hydrogen peroxide reacts with the acetic acid under the action of the solid acid catalyst to reform peracetic acid. An aqueous solution containing the peracetic acid thus reformed leaves the tank 6 by the pipe 3 and again joins the tank 1. The system is fed with fresh hydrogen peroxide and optionally with fresh acetic acid via the pipe 7. The system may be purged via the pipe 8.

EXAMPLE 1 (NOT IN ACCORDANCE WITH THE INVENTION)

[0047] A synthetic solution of benzothiophene (BT) and dibenzothiophene (DBT) in toluene is used to simulate a mixture of hydrocarbons containing sulphur-containing derivatives. The benzo- and dibenzothiophene type compounds are difficult to remove by hydrodesulphurization and predominantly contribute to the S content of certain petroleum products.

[0048] 80 g of a solution of benzothiophene (BT, 5.249 g/kg) and of dibenzothiophene (DBT, 7.188 g/kg) in toluene, which corresponds to an S content of 2 500 ppm by weight, 9.61 g of acetic acid and 12.25 ml of a solution of hydrogen peroxide at 37.8% by weight are introduced into a jacketed thermal glass reactor equipped with a paddle, mixer and surmounted by a condenser cooled to −20° C. The mixture is stirred for 1 h at 25° C., and then for 4 h at 50° C.

[0049] The BT and DBT contents are determined by gas chromatography after 5 h.

[0050] After 5 h of reaction, the residual BT and DBT contents correspond to an S content of 1 211 ppm weight.

EXAMPLE 2 (NOT IN ACCORDANCE WITH THE INVENTION)

[0051] The conditions of Example 1 are, reproduced except that 0.21 g of H₂SO₄ at 97% is introduced.

[0052] After 5 h of reaction, the residual BT and DBT contents correspond to an S content of 14 ppm weight.

EXAMPLE 3 (IN ACCORDANCE WITH THE INVENTION)

[0053] The conditions of Example 1 are reproduced except that 2.25 g of Nafion® NR 50 (beads 7-9 mesh, Aldrich) are introduced.

[0054] After 5 h of reaction, the residual BT and DBT contents correspond to an S content of 107 ppm weight.

EXAMPLE 4 (IN ACCORDANCE WITH THE INVENTION)

[0055] The conditions of Example 1 are reproduced except that 2.25 g of a zeolite H-ZSM-5 (Zeochem, CU Chemie Uetikon AG, ZEOCAT PZ-2/25 H, Si/Al=17) are introduced.

[0056] After 5 h of reaction, the residual BT and DBT contents correspond to an S content of 910 ppm weight.

EXAMPLE 5 (IN ACCORDANCE WITH THE INVENTION)

[0057] The conditions of Example 1 are reproduced except that 2.25 g of a zeolite H-MOR (Zeochem, CU Chemie Uetikon AG, ZEOCAT FM-8/25 H, Si/Al=12.5) are introduced.

[0058] After 5 h of reaction the residual BT and DBT contents correspond to an S content of 803 ppm weight.

EXAMPLE 6 (IN ACCORDANCE WITH THE INVENTION)

[0059] The conditions of Example 1 are reproduced except that 0.41 g of Dowex® 50 W X 8 (beads 20-50 mesh, H⁺-form, Fluka) is introduced.

[0060] After 5 h of reaction the residual BT and DBT contents correspond to an S content of 324 ppm weight.

Example 7 (IN ACCORDANCE WITH THE INVENTION)

[0061] The conditions of Example 1 are reproduced except that 2.25 g of particles of an inorganic solid (silica) grafted with organic acid groups, which is obtained according to the procedure described in the publication by J. H. Clark et al., C.R. Acad. Sci. Paris, Series IIc, Chimie/Chemistry 2000, 3, 399-404, are introduced.

[0062] After 5 h of reaction the residual BT and DBT contents correspond to an S content of 5 ppm weight.

Example 8 (IN ACCORDANCE WITH THE INVENTION)

[0063] The oxidation step was carried out on hydrotreated transport diesel fuel, the sulphur content of which is 39 ppm by weight measured by X-ray fluorescence. Analysis by gas chromatography with specific detection of sulphur by atomic emission (AED) shows that the sulphur compounds present are substituted dibenzothiophenes, and more particularly 4,6-dimethyldibenzothiophene.

[0064] 12.475 g of glacial acetic acid, 3.685 g of sulphonic resin Nafion® NR50 (beads 7-9 mesh, Aldrich), 132.565 g of the transport diesel fuel described above and 16.00 ml of an aqueous solution at 38% by weight of hydrogen peroxide are successively introduced into a jacketed thermal glass reactor provided with a paddle mixer made of glass and of a fluorinated polymer Teflon®, with a point for introducing nitrogen, with a condenser maintained at −25° C., and with a system for adding the hydrogen peroxide solution. The medium is kept at 25° C. for 1 h and then is heated at 50° C. for another 5 h. The phases are then separated and the organic phase is again washed with 3 times 25 ml of water. Analysis of the organic phase by gas chromatography with specific detection of the sulphur indicates a total conversion of the sulphur compounds present in the initial petroleum load.

[0065] A glass column having an inner diameter of 0.4 mm is filled with 523 mg of silica gel (Merck 60 grade silica, height of the adsorbent bed=8.7 cm). 41.662 g of the oxidized transport diesel fuel are passed over the adsorbent. Following this percolation, 40.627 g of diesel having an S content determined by X-ray fluorescence of 7 ppm weight are recovered. 

1. Process for the desulphurization of a mixture of hydrocarbons containing sulphur compounds, comprising a step of oxidation by means of hydrogen peroxide, acetic acid and an acid catalyst in order to oxidize the sulphur compounds, in which the acid catalyst contains an acid solid chosen from cation-exchange resins having a pKa of less than or equal to 4, from solids based on silicon oxide additionally containing a trivalent metal, and from particles of inorganic solid on which acidic organic groups are grafted such that the particles thus grafted have a pKa of less than or equal to
 4. 2. Process according to claim 1, characterized in that the acid catalyst contains a cation-exchange resin having a pKa of less than or equal to 4 and is chosen from fluorinated resins where the hydrogen atoms of the hydrocarbon backbone have been partially or completely substituted with fluorine atoms and which contain sulphonic and/or carboxylic groups.
 3. Process according to claim 2, characterized in that the acid catalyst contains particles of resin which are dispersed in an inorganic matrix.
 4. Process according to claim 2 or 3, characterized in that a quantity of acid catalyst is used such that the number of mol of protons from the carboxylic and/or sulphonic groups per kg of aqueous phase with which the catalyst is in contact is from 0.001 to 0.8.
 5. Process according to claim 1, characterized in that the acid catalyst contains a solid based on silicon oxide additionally containing aluminium and in which the silicon/aluminium molar ratio in the solid is from 3, to 200, and in that the solid is chosen from zeolites, clays and amorphous solids based on silicon oxide additionally containing aluminium.
 6. Process according to claim 5, characterized in that the solid contains sulphonic and/or carboxylic functionalities.
 7. Process according to claim 5 or 6, characterized in that a quantity of acid catalyst is used such that the number of mol of aluminium per kg of aqueous phase with which the catalyst is in contact is from 0.001 to 0.8.
 8. Process according to claim 1, characterized in that the acid catalyst contains particles of inorganic solid on which acid organic groups are grafted such that the particles thus grafted have a pKa of less than or equal to 4, the inorganic solid being silicon oxide and the acid organic groups being chosen from carboxylic and/or sulphonic groups.
 9. Process according to claim 8, characterized in that a quantity of acid catalyst is used such that the number of mol of protons from the carboxylic and/or sulphonic groups per kg of aqueous phase with which the catalyst is in contact is from 0.001 to 0.8.
 10. Process according to any one of the preceding claims, characterized in that the oxidation is carried out at a temperature of 0 to 100° C. and using a quantity of hydrogen peroxide such that, in the oxidation reaction medium, the molar ratio of the hydrogen peroxide relative to the sulphur present in the mixture of hydrocarbons is from 1 to 5 000, a quantity of acetic acid such that, in the oxidation reaction medium, the molar ratio between the acetic acid and the hydrogen peroxide is 0.01 to 4, and in that a volume of aqueous phase essentially containing water, hydrogen peroxide, part of the acetic acid and part of the peracetic acid formed in situ is brought into contact with the mixture of hydrocarbons such that the ratio of the volumes of this aqueous phase and of the mixture of hydrocarbons is from 0.01 to 0.5.
 11. Process according to any one of the preceding claims, characterized in that the oxidation is preceded by one or more hydrodesulphurization steps and in that the oxidation is followed by one or more steps of separating the oxidized sulphur compounds. 